Most ophthalmic lenses dispensed today are made from plastic materials, such as diethylene glycol bis(allylcarbonate), a common form thereof being sold under the trademark "CR-39" by PPG, or other allylbased optical polymers. Plastic lenses are inexpensive to fabricate yet possess good optical properties and are lighter in weight than glass lenses. In the manufacture of plastic lenses it is often desirable to incorporate optically useful components, such as polarizing films, or films containing various dyes for different purposes, for example, to provide photochromic properties. Such films are well-known in the art and are generally formed as thermoplastic polymers, an example of which is a polymer film formed from polyvinyl alcohol. Typically, these films are either laminated between two formed lens blanks using an adhesive such as polyurethane or optical-grade epoxy, or a film previously coated with polyurethane is sandwiched within a mold filled with monomer that is then reacted to encapsulate the film and form a complete lens blank. The result is the formation of either a laminated or cast lens blank, manufactured with different diopters, and then shipped to optometrists and/or optometric laboratories where the lenses are ground to prescription and fit to various eye glass frames.
A significant problem that arises with the formation of such laminated lens blanks is a propensity for such blanks to shear when subjected to shock or when ground improperly. Shearing occurs along the interface of the polymer film and the main polymeric substrate; in effect, if the polymer film is only weakly bonded to the blanks, it acts somewhat as a release agent for the laminated substrate components when under stress. Similarly, this interface can be a weak spot even in cast blanks.
Another problem found with current adhesives is their limited shelf life before becoming unusable, as short as 48 hours. Additionally, current adhesives tend to seep, decreasing mold life. Moreover, in existing laminated lenses, the polymer film tends to yellow, presenting varying color lens to lens, or over time.